In the distribution of natural gas to customers, gas distribution companies provide a main gas line while maintaining the pressure of the gas at about 20 to 60 pounds per square inch (PSIG). Since most home appliances are programmed to operate under gas pressures of about 1/2 PSIG, the gas pressure directed from the main gas line to each home must be reduced to this lower pressure, i.e. 1/2 PSIG. To accomplish this, pressure regulators are placed in the gas line to reduce the gas pressure from about 20 to 60 PSIG to 1/2 PSIG to make it suitable for customer use. These pressure regulators are either located at the service entrance for each individual customer or in a district regulator station that serves a number of customers. To protect against overpressurization of the gas supplied to customers because of regulator failure, a safety device is required downstream of the pressure regulator. This safety device is located between the customers+ gas lines and the pressure regulator. These safety devices can be installed to act at either individual service regulators or at district pressure regulators. Individual service regulators are equipped with internal relief valves which automatically vent any excess gas (beyond about 1/2 PSIG) to the atmosphere. The internal relief valves used are simple, reliable spring-operated devices similar in design to the pressure relief valve located on hot water tanks.
District pressure regulators on the other hand are usually larger than individual service regulators. There are a number of different devices that have been used to provide overpressure protection. These include safety shut off valves, monitor regulators and relief valves and all are generally located downstream of the district pressure regulator. The safety shut off valve will shut off gas flow in the event of a regulator failure and therefore are impractical since their use requires relighting every customer's appliances. The monitor regulator's function is to take over the pressure reduction activity in the event of district pressure regulator failure. The monitor regulators have substantially the same mechanical structure as district pressure regulators. A problem with the use of monitor regulators is that they suffer from the same failure cause as the district pressure regulator. Thus, dirt or other debris passing through the piping system is likely to have the same adverse effect on both the district pressure regulator and the monitor regulator. This can result in the failure of both devices and subsequent overpressurization of the downstream system.
Relief valves are provided to sense the downstream pressure in a piping system and are designed to open when the pressure exceeds a predetermined setpoint. In a stable and normal operating mode these relief valves are in a closed position and no gas flows through them. Upon overpressurization the relief valve opens and excess gas is vented to the atmosphere. It is common to install relief valves with a setpoint of 2/3 PSIG, and when the district pressure regulator fails to keep the pressure below this 2/3 PSIG setpoint, the relief valve opens to vent the gas. Every district pressure regulator has an associated maximum capacity. Given a maximum inlet pressure, it is possible to calculate the peak gas volume that can pass through a district pressure regulator. The relief valve associated with a district pressure regulator must have a greater capacity than the regulator. As long as this size relationship is maintained, the relief valve will have the ability to vent all of the excess gas that the district pressure regulator is capable of allowing into the system. For this reason relief valves generally have a greater diameter than their associated regulators. It is common to install a four or six inch diameter relief valve downstream of a two inch diameter regulator. The use of a relief valve as the safety device in this type system appears to be the most practical of the prior art devices. The present invention relates to a system and a safety device utilizing a novel pressure relief valve configuration.
There are three basic types of relief valves, liquid sealed, self operated and pilot operated. In each case, in a stable system, gas is restrained by a mechanical sealing mechanism. Liquid sealed relief devices are the simplest of the three. A large tank of liquid, usually an oil similar to motor oil, is placed near the piping system downstream of a district pressure regulator. A branch line from the downstream system is run to the top of the tank. The branch line is turned downward with the open end of the line ending below the surface of the liquid. As long as the head pressure of the liquid is greater than the gas pressure in the line, the system stays sealed. If the gas pressure exceeds the head pressure of the liquid, it forces the liquid out of the tank and allows the gas to flow to the atmosphere. Setpoint can be controlled by varying the level of the liquid in the tank. Liquid seal relief devices are very messy. They are not appropriate in an environmentally conscious society. They also have the disadvantage that they will not reseal themselves if system pressures return to normal. They will continue to allow gas to flow to the atmosphere until someone refills them with the liquid.
In self operated relief valves a plug and orifice combination is common. They are configured much the same as a standard water faucet with the water sealed behind the orifice by a rubber plug. However, instead of forcing the plug into place with a threaded, manually operated stem, a series of weights or a spring holds the plug down. By carefully controlling the amount of weight or spring compression holding the plug into place, it is possible to design a device to open at any desired internal system pressure. Self operated relief valves open when the pressure per square inch acting upward on the area on the bottom of the plug exceeds the down force generated by the weights or spring compression attempting to hold the plug down. Self operated relief valves do have the advantage that they will reseal themselves once system pressures return to normal.
Self operated relief valves have an inherent problem called build up. There is a relationship between setpoint, the volume of gas that needs to be vented and the amount of plug movement required to vent that gas. When the system pressure is equal to setpoint, a state of equilibrium exists. A slight increase in system pressure results in a slight upward movement of the relief valve's plug. It follows that a significant movement of the valve plug requires a significant increase in system pressure. This increase is called build up. It is common to design a self operated relief valve to have a set point of 2/3 PSIG and allow a build up to 2 PSIG before the desired maximum capacity is achieved.
Conventional prior art pilot operated relief valves use gas pressure to hold the sealing mechanism in place. The piping system being protected has traditionally been the source of this pressure. The down force required to keep these valves closed had been generated by designing a valve where the underside of the plug had a smaller area than the upper side. The introduction of the same pressure per square inch to both sides of such a plug results in a net down force proportional to the difference in the two surface areas. This same basic relationship has been used regardless of the sealing mechanism. This difference in surface areas approach has been used to operate sleeve type and piston type valves.
In high pressure applications a slight difference in surface areas results in a significant net sealing force. In low pressure applications a much larger difference in surface areas is required to achieve an acceptable net sealing force. This has led low pressure relief valve designers to use complex and expensive castings to create functional pilot operated relief valves.
There are several known systems for conveying and supplying natural gas. There are also, as noted earlier, alternate means for controlling overpressurization of the gas along the supply lines. In U.S. Pat. No. 323,840 (Westinghouse I) a method of conveying gas is disclosed wherein a low gas pressure is maintained in the pipelines to reduce the tendency of leakage or rupture of the pipes. Westinghouse I does not address the problem of automatically controlling overpressurization with a safety device such as a relief valve that operates in cooperation with an inspirator. Westinghouse I is more concerned with conveying gas by exhaustion from one to another section of a conducting main in which an average pressure below that of the atmosphere is maintained. In U.S. Pat. No. 328,368 (Westinghouse II) a process for reducing pressure by an exhausting device such as a reciprocating or rotary pump or blower is disclosed. This exhausting device is driven by steam, compressed air or high pressure gas. The gas is conveyed in pipes or a jet apparatus activated by high pressure gas located adjacent to the delivery end of each of separate compartments. The exhausted gas is forced through a pipe into the receiving end of the next succeeding compartment thus effecting a reduction in pressure. In U.S. Pat. No. 4,622,999 (Ray) a gas flow control system which utilizes a pilot control of the main gas valves and a boosted gas pressure as the motive fluid is disclosed. Ray's system comprises a main diaphragm control valve, a second similar valve which is a diaphragm operated shut off valve and a third diaphragm valve which is in a vent line connected to the line between the other two valves. A booster pump is provided in Ray's system which draws on the incoming gas itself and boosts its pressure so that its discharge pressure can be used for operation of the valves and all of the other components of the system. Ray's system does not utilize an auxiliary gas line to operate the venting means nor does Ray suggest the use of an inspirator to effect a vacuum which activates the gas venting means.